For elevator systems it is known that a door assembly of an elevator car may include a high speed and a slow speed door. A two speed door mechanism interconnects the doors which controls their speed relative to each other. Currently two types of mechanisms are used to open and close the two speed door assembly. One type of mechanism uses multiple links to interconnect the low speed door, the high speed door and a drive motor. This mechanism requires a number of components which increase the time and cost to manufacture and assemble the door assembly.
Another two speed door mechanism uses cables and pulleys to control the operation of the doors. The cable design has proven to have inadequate stiffness necessary for smooth two door operation. The cable design also forces the mechanism to be installed in the header of the elevator car which is not possible for some elevator systems that do not have space in the header area for a cable arrangement.
In addition to controlling the movement of the doors, the door mechanism must also hold the doors closed at a constant force to comply with elevator codes. One current method is the use of a motor to provide constant force to the door assembly in the closed position. The use of motor force is undesirable because it consumes energy and provides additional heat to the system. Another method is the use of weight closers which provide a constant weight or force closing the door assembly. This method, however, is undesirable because the weights exert a constant closing force not only at the closed position but also at all positions. The driving motor must therefore continually overcome this constant closing force throughout the opening of the doors and thereby consume energy and provide additional heat to the system. Another method is the use of springs to provide a constant closing force when the door assembly is closed. Springs, however, can be noisy and introduce reliability problems.